Boiler short cycling

After draining following Sentinel x400, and refilling, boiler fires for 10 to 15 secs as temperature rises quickly, then shuts off. It refires after a few minutes. This is occurring when putting on central heating or hot water. Boiler seems to work normally if only the underfloor heating is on. If its air in the system causing the problem, then why it is ok when only underfloor heating is on. What is the best way to deal with this? Grateful for any suggestions.

 

Thanks. Yes probably air. Maybe ufh system pipes are bigger diameter and less troubled by air.